1. Field of the Invention
The present invention relates to a pigment dispersion and an inkjet ink using the pigment dispersion. In addition, the present invention also relates to a method for preparing the pigment dispersion and the inkjet ink; an ink set using the inkjet ink; an ink cartridge containing the inkjet ink; an image forming method and apparatus using the inkjet ink; and an image produced by the image forming method and apparatus.
2. Discussion of the Background
Inkjet recording methods have the following advantages over other recording methods:
(1) The recording process thereof is relatively simple compared to other recording methods and therefore full color images can be easily produced; and
(2) even when a printer having a simple constitution is used, high-resolution color images can be produced.
Dye-based inkjet inks in which a water-soluble dye is dissolved in water or a mixture solvent of water and an organic solvent have been typically used as inkjet inks. However, dye-based inkjet inks have a drawback in that the resultant images have poor light resistance, although the dye-based inkjet inks can produce color images having good chromaticness. Therefore, pigment-based inkjet inks in which one or more pigments such as carbon black and various organic pigments are dispersed have been investigated because of having a relatively good light resistance compared to the dye-based inkjet inks.
However, pigment-based inkjet inks have a drawback of frequently causing a clogging problem in that inkjet nozzles are clogged with the inks, resulting in formation of images having omissions and/or low density. In addition, pigment-based inkjet inks have a drawback in that the color gamut of images produced using the pigment-based inkjet inks is less than that of the images produced by dye-based inkjet inks.
Recently, a need exists for an inkjet ink in which a colorant having a small particle diameter is dispersed, to produce high resolution images. However, the smaller the pigment in an inkjet ink, the more frequently the inkjet ink (i.e., the pigment) is aggregated, resulting in deterioration of discharging stability of the inkjet ink. Therefore, an inkjet ink which includes a colorant having a small particle diameter and which has good discharge stability has not yet been developed.
In order to prevent the clogging problem and improve the color gamut of a pigment-based inkjet ink, the average particle diameter of the colorant of the ink has to be reduced. Therefore, conventional inkjet inks use a polymer-dispersant such as styrene-acrylic acid copolymers, styrene-maleic acid copolymers (disclosed in Unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 56-147863, etc.), and sodium naphthalenesulfonate-formalin condensation products (disclosed in JP-A 61-083267, etc.); or which use a surfactant such as polyethyleneglycol alkylphenyl ethers (disclosed in JP-A 05-105837, etc.), sulfates of polyethyleneglycol alkylphenyl ethers (disclosed in JP-A 10-168367, etc.), and phosphates of polyethyleneglycol alkylphenyl ethers (disclosed in JP-A 10-88050, etc.). However, each of such inkjet inks has a large average particle diameter, and in addition the inkjet inks have poor discharge stability and preservation stability. In addition, the images produced by the inkjet inks have poor clearness.
Because of these reasons, a need exists for a pigment-based inkjet ink which has good discharge stability and a preservation stability and which can produce images having good clearness.
Accordingly, an object of the present invention is to provide a pigment dispersion for use in a pigment-based inkjet ink which has good discharge stability and a preservation stability and which can produce images having good clearness (high chromaticness).
Another object of the present invention is to provide an inkjet ink, an inkjet ink set, an ink cartridge, and an image forming method and apparatus, by which color images having good light resistance and good clearness (high chromaticness) can be produced without causing the clogging problem.
Yet another object of the present invention is to provide methods for preparing the pigment dispersion and the inkjet ink.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a pigment dispersion including:
a pigment;
a dispersant; and
a dispersion medium including water,
wherein the pigment is dispersed in the dispersion medium, wherein the pigment has an average particle diameter D50 not greater than 50 nm and a pigment particle diameter distribution, and wherein a standard deviation of the pigment particle diameter distribution is less than the average particle diameter D50.
The dispersant is preferably included in the pigment dispersion in an amount of from 0.3 to 0.2 parts by weight per 1 part by weight of the pigment.
The pigment preferably includes a compound having the following formula (1): 
wherein n is an integer of from 20 to 100. More preferably n is 40.
It is preferable that the pigment includes one of Pigment Red 122, Pigment Blue 15:3, and Pigment Yellow 138.
As another aspect of the present invention, a method for preparing a pigment dispersion is provided which includes:
mixing a pigment, a dispersant, and a dispersion medium including water, wherein the weight ratio (P/D) of the pigment (P) to the dispersant (D) is from 1/0.3 to 1/2; and
subjecting the mixture to a wet dispersion treatment to prepare a pigment dispersion, wherein the pigment dispersion is the pigment dispersion mentioned above.
As yet another aspect of the present invention, an inkjet ink is provided which includes the pigment dispersion, and one or more additives such as water, water-soluble organic solvents and surfactants. A fluorine-containing surfactant is preferably added thereto.
As a further aspect of the present invention, a method for preparing an inkjet ink is provided which includes mixing the above-mentioned pigment dispersion and at least one of water, a water-soluble organic solvent and a surfactant.
As a still further aspect of the present invention, an inkjet ink set is provided which includes plural inkjet inks having different color tones, wherein each of the plural inkjet inks is the inkjet ink of the present invention.
As a still further aspect of the present invention, an ink cartridge is provided which includes a container and an ink which is contained in the container, wherein the ink is the inkjet ink of the present invention.
As a still further aspect of the present invention, an inkjet printer is provided which includes:
an ink cartridge containing an ink; and
an inkjet print head configured to discharge the ink to form an image on a receiving material,
wherein the ink is the inkjet ink of the present invention.
As a still further aspect of the present invention, an inkjet printing method is provided which includes:
discharging an ink from a nozzle to form an image on a receiving material,
wherein the ink is the inkjet ink of the present invention.
As a still further aspect of the present invention, an image which is produced on a receiving material by the image forming method or apparatus is provided. The receiving material is preferably a paper.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.